In the manufacture of a wide variety of products, especially molded products, chopped fiber (e.g. glass fiber) mats are used in the molding operation and typically saturated with resin. These mats have conventionally been produced by air laid techniques, at a production rate that is normally between about 20-30 m/min., and must be relatively thick/dense otherwise they have too many holes and discontinuities to be fully effective in molding on other subsequent processing operations. These mats are typically made of fiber bundles having five or more fibers per bundle, typically about 10-450 fibers/bundle.
Glass tissue produced by the wet laid method or by the foam method comprises individual fibers or fiber bundles with very few (typically less than five) fibers in a bundle. Sometimes, some fiber bundles have not dispersed fully into the slurry. These poorly dispersed fiber bundles are elongated bundles, because the individual fibers of the bundle have slid with respect to each other. The length of an elongated fiber bundle is much longer than the length of the individual fibers. The fiber bundles that enter the slurry formation process comprise fibers that have the same length as the fiber bundle, since the yarn (typically about 10-450 fibers) is cut into bundles having a predetermined length in cutters. Elongated fiber bundles are defects in the fiber tissue, causing an uneven surface configuration of the tissue. In a poor quality glass tissue, there may be as much as about 5-10% elongated fiber bundles.
Exemplary prior art techniques for making glass fiber mats by the air laid method and making glass fiber tissue by the wet laid method are described in K. L. Loewenstein: The Manufacturing Technology of Continuous Glass Fibres, 1993 (incorporated by reference herein).
According to the present invention the limitations of the prior art mats described above are substantially overcome or minimized by employing one or more simple yet effective techniques. According to the present invention preferably the fibers are held in the bundles with a non-water soluble sizing, such as epoxy resin or PVOH, and/or 5-450 (e.g. about 10-450) fibers are provided in each bundle, each fiber having a diameter of about 7-500 microns, preferably about 7-35 microns, and at least about 85% of the fibers have a length of 5-100 mm, preferably about 7-50 mm (and all narrower ranges within these broad ranges).
According to the invention it is possible to produce mats having a substantially uniform density yet can be of much lower density than can be produced using air laid techniques. For example, mats can be produced having a density as low as 50 gm/in2, or even less. The mats may be produced much more rapidly than by air laid techniques, and a wider variety is possible. For example, mats having multiple layers of different physical properties and/or compositions may readily be produced. These advantageous results are accomplished by using a water or foam laid process, so that production speeds of well over 60 m/min. (typically over 80 m/min, e.g. about 120 m/min.) are readily achieved, along with highly uniform mats of a wide variety of constructions. Utilization of the foam process is preferred, however, for many reasons, including process efficiency. Using the foam process the slurry can have 0.5-5% (or any smaller range within that broad range) fibers by weight, whereas in the wet laid process the maximum fiber content is about 0.05% by weight. If a larger percentage of fibers is used in the wet laid process then the viscosity of the liquid must be increased (by introducing additives), and that causes several problems, including the formation of air bubbles. This would require still further additives, making the wet laid process much more difficult and expensive compared to the foam process.
According to one aspect of the present invention there is provided a non-woven mat of chopped strands, comprising: A plurality of fibers disposed in a non-woven configuration to define a mat. At least 20% of the fibers in fiber bundles having between 5-450 fibers per bundle and the length of the bundles being substantially the same as the lengths of the fibers forming the bundles, and wherein at least 85% of the fibers of the fiber bundles have a diameter of between about 7-500 microns.
Preferably at least 85%, up to substantially 100%, of the fibers in the bundles have a length of between 5-100 mm, preferably 7-50 mm, most preferably between about 20-30 mm, and at least 50%, preferably at least 85% of substantially 100%, of the fibers in the bundles have a diameter of between 7-35 microns. Typically the fibers in the fiber bundle are held together with a substantially water insoluble sizing, such as epoxy resin or PVOH. Preferably substantially all of the fibers in a bundle are substantially straight.
The invention is particularly useful where at least 10% (preferably at least about 50%, up to substantially 100%) of the fibers in fiber bundles comprise reinforcement fibers selected from the group consisting essentially of glass, aramid, carbon, polypropylene, acrylic, and PET fibers, and combinations thereof. The invention is particularly suitable for use with glass fibers.
By practicing the invention it is possible to make mats with an extremely wide density range, e.g. between about 50-900 g/m2, yet with substantially uniform density. For example, the mat may have a substantially uniform density of less than 75 g/m2 (even below 50 g/m2 depending the fibers utilized). When the mat has a density between about 50-150 g/m2, 90% of the fibers in the fiber bundles have between 10-200 fibers per bundle. Typically at least 85% of the fibers in the fiber bundles have between 10-450 fibers per bundle and a length substantially the same as the length of the fiber bundle.
According to another aspect of the present invention a method of producing a non-woven chopped strand mat is provided comprising: (a) Forming a slurry of fibers in a liquid or foam (preferably foam) wherein at least 20% of the fibers in the slurry are in fiber bundles in which the fibers are held in the bundles by a substantially non-water soluble sizing. (b) Forming a non-woven web from the slurry on a foraminous element. And (c) withdrawing at least one of liquid and foam from the slurry on the foraminous element so as to form a non-woven mat. Preferably the slurry in (a) has between about 0.5-5% by weight fibers. The liquid process practice may be entirely conventional, and the foam process practice may be such as shown in U.S. Pat. No. 5,904,809, issued May 18, 1999 (the disclosure of which is hereby incorporated by reference herein). The invention also relates to products made from this method.
Because the invention uses a liquid or foam process as opposed to air laid process, the speeds of production are much greater. That is, (b) and (c) may be practiced at a speed of at least 60 m/min, typically at least 80 m/min, and may easily achieve speeds of 120 m/min. The foraminous may have any suitable conventional construction such as a conventional wire, or dual or multiple wires, etc. For example (a)-(c) may even be practiced using a moving web of fabric which becomes part of the mat produced as the foraminous element (or one of a plurality of such elements). Also by utilizing the invention (particularly such as by utilizing a segmented head box, such as shown in copending application Ser. No. 09/255,755, filed Feb. 23, 1999, the disclosure of which is incorporated by reference herein, or U.S. Pat. No. 4,445,974.
In the method typically (a) forming a slurry of fibers in a liquid or foam (preferably foam) wherein at least 20% of the fibers in the slurry are in fiber bundles in which the fibers are held in the bundles by a substantially non-water soluble sizing; (b) forming a non-woven web from the slurry on a foraminous element; and (c) withdrawing at least one of liquid and foam from the slurry on the foraminous element so as to form a non-woven mat. For example (a) is practiced using at least 10% (for example at least 50%, and at least 85%, up to substantially 100%) of reinforcing fibers in the fiber bundles, the reinforcing fibers selected from the group consisting essentially of glass, acrylic, aramid, carbon, polypropylene, and PET fibers, and combinations thereof. Also, (a)-(c) may be practiced so as to produce a mat having a substantially uniform density of between about 50-150 gm/m2.
The method may further comprise producing a second mat from at least a second slurry having a different fiber composition or density than the slurry from (a), and laying the at least a second slurry in a substantially non-mixing manner on the slurry from (a) to produce a composite mat having at least two substantially distant layers with different fiber compositions or densities. Alternatively or in addition the method may further comprise (d) providing at least one surface layer on the mat and affixing the at least one surface layer to the mat with a binder. The method typically further comprises curing the binder from (d) and drying the web in a drying oven. For example (a) is further practiced using heat activated binder powder or fibers in the slurry.
According to another aspect of the present invention there is provide a method of producing a non-woven chopped strand mat comprising: (a) Forming a slurry of fibers in a liquid or foam wherein at least 20% of the fibers in the slurry are in fiber bundles having between 10-450 fibers/bundle and a length substantially the same as the length of said fiber bundle, which length is between 5-100 mm for at least 85% of the fibers in bundles, and a diameter of the fibers in bundles of between 7-500 microns. (b) Forming a non-woven web from the slurry on a foraminous element. And (c) withdrawing at least one of liquid and foam from the slurry on the foraminous element so as to form a non-woven mat. The details of this aspect of the invention are preferably substantially as described above.
According to another aspect of the present invention there is provided a composite product comprising outer layers made from resin impregnated and cured mats as described above and an inner layer of at least one of inexpensive fibers, scrap fibers, and material of significantly lower density than said outer layers. A fiber based web may be manufactured from the foam process comprising at least two layers (or parts of layers) with different physical or chemical properties.
The invention also relates to a non-woven fibrous composite web manufactured by using a liquid or foam based process using a xe2x80x9cmulti-layer headboxxe2x80x9d and/or xe2x80x9cdivided headboxxe2x80x9d, having at least two layers having substantially different properties, including at least one of different density, different material, different reinforcement threads, and different reinforcement webs. The composite web may comprise threads or webs of substantially continuous fibers and with directional properties, e.g. reinforcement threads and webs with directional strength properties that are fed to the web through the headbox. At least a part of the composite web may comprise a heat-activated binder in a powder form or in a fibrous form. At least 20% (e.g. at least 40%) of the fibers fed to a headbox may be attached to each other to form fiber bundles by using some appropriate hydrophobic sizing-agent such as epoxy resin or PVOH. Preferably the length of the fibers in a fiber bundle is substantially the same as the length of the fiber bundle, and the number of fibers in a fiber bundle is variable and preferably between about 10-450 fibers, and the length of the fibers in a fiber bundle is about 5-100 mm, preferably about 7-50 mm. At least on one side of the composite non-woven web there may be at least one surface layer of fabric that is attachable to the non-woven composite web by binders on the surface of the fabric or on the web in a drying oven (or the like) positioned after the web-formation apparatus (headboxes).
According to the present invention all narrower ranges within the broad ranges set forth above are specifically provided herein. For example, the diameter of the fibers in the bundles of between 7-500 microns comprises 9-450 microns, 10-30 microns, 9-300 microns, and all other narrower ranges within the broad range specified.
It is the primary object of the present invention to provide a highly advantageous mat, products made from the mat, and a method of production of the mat, that overcome a number of the problems in the prior art chopped glass fiber mat and glass tissue arts. This and other objects of the invention will become clear from a detailed description of the invention and from the appended claims.